It is well known that graphite consists of many sp2-hybridized carbon sheets stacked on top of one another. When graphite is exfoliated into a few-layer structure, the individual graphitic sheets are collectively referred to a material known as graphene. Graphene typically refers to a material having less than about 10 graphitic layers. The graphitic layers are characterized by a two-dimensional basal plane having a hexagonal lattice structure. In many cases, various edge and/or basal plane functionalities such as, for example, carboxylic acid groups, hydroxyl groups, epoxide groups and ketone groups are also present due to oxidation that either occurs natively in graphite or during exfoliation. Oxidative damage may also manifest itself in the form of defects (i.e., holes) in the basal plane.
Graphene nanoribbons are a special class of graphene, which are similarly characterized by a two-dimensional basal plane, but with a large aspect ratio of their length to their width. In this regard, graphene nanoribbons bear similarity to carbon nanotubes, which have a comparable aspect ratio and are defined by one or more layers of graphene sheets rolled up to form a cylinder. Graphene nanoribbons possess a number of useful properties, including, for example, beneficial electrical conductivity. Unlike carbon nanotubes, which can be metallic, semimetallic or semiconducting depending on their chiral geometry and diameter, the electrical properties of graphene nanoribbons are governed primarily by their width. For example, graphene nanoribbons less than about 10 nm wide are semiconductors, whereas similar graphene nanoribbons having a width greater than about 10 nm are metallic or semimetallic conductors. The edge configurations of graphene nanoribbons having an “armchair” or “zigzag” arrangement of carbon atoms, along with any edge functional groups, also may affect the transmission of electron carriers. Such “armchair” and “zigzag” arrangements are analogous to those defined in the carbon nanotube art. Ballistic charge transport in graphene and graphene nanoribbons drops markedly if the sp2 network of the graphene basal plane is disrupted by even a relatively small number of defects.
Various methods for making graphene sheets are known, including, for example, adhesive tape exfoliation of individual graphene layers from graphite, chemical-based exfoliation of graphene layers from graphite, and chemical vapor deposition processes. Each method provides on the order of picogram quantities of graphene. Several lithographic and synthetic procedures have been developed for producing minuscule amounts of graphene nanoribbons. Microscopic quantities of graphene nanoribbons have been produced by partial encapsulation of carbon nanotubes in a polymer, followed by plasma etching to longitudinally cut the carbon nanotubes. Macroscopic quantities of graphene nanoribbons have been produced by a chemical vapor deposition process or by an oxidative process in concentrated acid.
In addition, multi-walled carbon nanotubes (MWNTs) have been non-selectively opened by intercalation and reaction with lithium in liquid ammonia solvent, resulting in longitudinal carbon nanotube opening to produce multilayered graphitic structures including partially opened MWNTs, graphene flakes, and graphene nanoribbons functionalized with hydrogen. Graphene nanoribbons prepared by these processes are typically characterized by multiple graphene layers with a kinked morphology and a defect-prone atomic structure having various oxygenated functionality, because the MWNT starting material has to be oxidatively damaged to provide sites for the lithium-ammonia reaction to occur. Although oxygenated functionality in graphene nanoribbons can be largely removed by subsequent reduction, defects in the graphene basal plane are not repaired by reduction, and the conductivity does not approach that of pristine graphene. While the lithium intercalation method is reductive and does not introduce oxygenated functionality into the graphene nanoribbons, defect-free graphene nanoribbons are not produced by this method due to the initial introduction of defects into the MWNTs, which are then carried forward into the graphene nanoribbon product.
In view of the foregoing, methods for preparing graphene nanoribbons having a substantially defect-free structure would be of considerable benefit in the art. Such defect-free graphene nanoribbons may demonstrate considerable utility in electronic, mechanical and many other applications. Ideally, such methods would be scalable to produce macroscopic amounts of the graphene nanoribbons.